Current sense resistor circuit with average kelvin sense features

ABSTRACT

A current sense resistor circuit using Kelvin connection sense features provides an average voltage across net sense resistance and average voltage across net reference resistance to be available at the Kelvin connection points. The Kelvin connections can be used by a negative feedback gain loop to hold the average current through respective reference elements and sense elements substantially constant.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO AN APPENDIX

Not applicable.

BACKGROUND

1. Technical Field

The technology described herein is generally related to the field ofelectrical circuits.

2. Description of Related Art

Electrical current sensing is used in a variety of applications. Forexample high current demand apparatus, such as motors, generally havecontrol circuits which use current sensing to implement control signalsback to the load. FIG. 1 (Prior Art) is an electrical schematic diagramof a conventional current sense circuit 100 for setting a voltage levelto a predetermined value for such apparatus operations. For example, andreferring to FIG. 1 (Prior Art), measuring a current is accomplished byforcing a current through a first, sense resistor “Rsense” and measuringthe voltage “V” across the sense resistor, where V_(MEASURED)=i₁*Rsense.In control circuitry, to control the current “i₁” through the resistorRsense, a feedback gain loop can be used to measure the voltage acrossthe resistor and then adjust the current “Iset” so that the voltage isheld at a predetermined value. An operational amplifier, “OP AMP,” 101is used to represent any negative feedback gain component(s) needed toforce the electrical current Iset through the sense resistor Rsenseconnected in parallel with a reference resistor, “Rref,” thereby forcingthe same voltage drop across both resistors, Rsense, Rref. The secondresistor Rref has a reference current “Iref” flowing through it. Thus,the sensed current is the value of “Iout”:Iout=(Rref−Rsense)×Iref.The ratio of the sizes of the two resistors is known to be controllablein monolithic silicon processing. However, for large currents, lout, thevalue of Rsense has to be relatively small so as to limit the voltagedrop across Rsense; too large a voltage across Rsense often limits theworking voltage of the system. Therefore, to make the value of Rsenselow, Rsense has to be either made up of a physically large set of manyparallel resistors as shown in FIG. 2 (Related Art) or has to be madewith a material having inherent low resistance. If Rsense is madephysically large such as shown in FIG. 2, the metal interconnects,depicted as “Rconnect,” become quite resistive compared to the desiredvalue of Rsense and it becomes difficult to define the actual electricalresistance of Rsense. If a low resistance material is employed,especially if low compared to interconnect lines to Rsense, the actualelectrical resistance of Rsense again becomes difficult to defineaccurately. Another complication is that if Rref is made of the same lowvalue resistance material in order to provide accurate matching, Rrefthen generally needs to be physically large.

In addition to dealing with these problematical complications, anotheraspect of this disclosure relates to “Kelvin connections.” Kelvinconnections are used compensate for voltage losses caused by lineresistances which would otherwise cause errors in low voltagemeasurements. This is accomplished generally by providing a source lineand a measurement line—also referred to commonly as “force line” and“sense line,” respectively—to an interconnection point, known as theKelvin connection, which is as close to a testing device as possible.

BRIEF SUMMARY

The present invention generally provides for a current sense resistordevice having averaging Kelvin sense features.

The foregoing summary is not intended to be inclusive of all aspects,objects, advantages and features of the present invention nor should anylimitation on the scope of the invention be implied therefrom. ThisBrief Summary is provided in accordance with the mandate of 37 C.F.R.1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and moreespecially those interested in the particular art to which the inventionrelates, of the nature of the invention in order to be of assistance inaiding ready understanding of the patent in future searches.

(7) BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (Prior Art) is an electrical circuit diagram of a conventionalcurrent sense circuit, setting the voltage to the predetermined value.

FIG. 2 (Related Art) is an electrical equivalence circuit diagramassociated with FIG. 1 for illustrating problems associated with theprior art.

FIG. 3 in accordance with an exemplary embodiment of the presentinvention is an electrical diagram of a circuit unit for a current senseresistor layout using a local Kelvin sense node.

FIG. 4 illustrates a method for lowering current sense resistor netresistance value employing a plurality of circuit units as shown in FIG.3.

FIG. 5 is another exemplary embodiment of the present invention for thecircuit as shown in FIG. 4.

FIG. 6 is a circuit diagram illustrating a simplified schematic for thecircuit as shown in FIG. 5.

FIG. 7 is an illustration of a first fundamental aspect in accordancewith the exemplary embodiment as shown in FIG. 6.

FIG. 8 is an illustration of a second fundamental aspect in accordancewith the exemplary embodiment as shown in FIG. 7.

Like reference designations represent like features throughout thedrawings. The drawings in this specification should be understood as notbeing drawn to scale unless specifically annotated as such.

DETAILED DESCRIPTION

The present invention provides a method and apparatus for controls usingcurrent sensing. The present invention provides for sensing averagevoltage across a current sense resistor and a reference resistor using aKelvin connection. Sensing errors due to process gradients in thecurrent sense resistor, temperature changes across the current senseresistor, and the additive affects of interconnect metal resistance aresubstantially eliminated.

FIG. 3 is an electrical equivalent circuit diagram for an exemplaryembodiment of the present invention. The design is implemented toprovide a circuit unit 300 using a plurality of relatively physicallysmall individual sense resistors 301, 303, connected in parallel, suchthat they may be arranged in close proximity to each other. It isintended by the inventor that the present invention may be implementedin integrated circuit (IC) chips where, as it is well known, it isdesirable to pack many elements into microscopic regions of the chip.For best matching, it is preferred that the resistor units besubstantially the same size.

Shown in this exemplary implementation are two, series-connected,reference resistors 302, 304, Rref. The preferred embodiment may have aratio of “Rref/Rsense” of approximately 4:1 or greater. It will berecognized by those skilled in the art that for simplicity, thisschematic diagram shows only two sense resistors 301, 303 and tworeference resistors 302, 304, but, for a larger ratio of Ref/Rsense,more resistors may be added to these sets of the circuit unit 300; otherexemplary embodiments are described hereinafter. A local Kelvin currentsense connection node 306 is provided between the parallel senseresistors 301, 303, increasing the circuit's resistor-ratio accuracy.

The sense resistors 301, 303 have interconnects 305 _(T(TOP)), 305_(B(BOTTOM)) (metal, ceramic, doped semiconductor material, or the likeas would be known in the art), where “TOP” and “BOTTOM” serve only as arelative orientation with respect to the drawings for purpose ofdescription of the present invention, each interconnect also having aresistance—artificially represented here by symbols labeled“Rmetal”—which does not add significantly to Rsense resistance becauseof the physically small size per sense resistor unit. For example, aratio of Rref/Rsense again may be approximate 4:1. In other words, usingphysically relatively small sense resistors 301, 303, and relatively inclose proximity to each other, the resistance, Rmetal, of theinterconnects 305 _(T,B) is much less than the net resistance of Rsense:Rmetal<<Rsense.

It will be recognized by those skilled in the art that circuit unit 300design allows the ratio Rref/Rsense to be accurately controlled whereinthe interconnect resistance—net Rmetal—factor is negligible and can beignored. Because of the resultant relatively small size of circuit unit300, and by having resistors made with like material and havingrelatively same physical dimensions, this design greatly decrease errordue to process gradients across Rsense, Rref, and temperature changesacross Rsense, Rref and connection resistance, Rmetal.

As with FIG. 1, OP AMP 101 and interconnection lines in FIG. 3 andsubsequent FIGURES is representative of any negative feedback gaincomponent(s) employed to force the voltage across reference resistorsand sense resistors, thereby forcing the same voltage drop across both.

FIG. 4 is an exemplary implementation generally combining, in parallel,a plurality of circuit units 300 as shown in FIG. 3. This circuit 400implementation connects in parallel circuit units 300 ₁, 300 ₂ . . . 300_(N), thereby reducing the net Rsense and Rref resistance values,without changing the ratio Rref/Rsense (for simplification, whenreferring to all multiple elements shown with subscripts, a generic “ss”subscript is used, e.g. 300 _(ss)). In other words, having parallelcircuit units 300 _(ss) does not change the ratio Rref/Rsense. Eachcircuit unit 300 _(ss) is connected by an interconnect 406—having aneffective resistance “Rmetal_(TOP)” between each unit—interconnectingall local Kelvin current sense nodes 306 _(ss). Since Kelvin connectionsare used, the resistance Rmetal_(TOP) between each unit hassubstantially no effect. Therefore, variation of the voltage at thesense nodes 306 _(ss) will not cause the ratio Rref/Rsense currents tochange.

Similarly, because of the relatively high resistance value of thereference resistors 302 _(ss), 304 _(ss) as compared to the Rmetal atthe top of Ref and the top of Rsense, the current through R302 _(ss) andR304 _(ss) has negligible effect on the Kelvin voltage at node 306_(ss). However, in such a configuration, the resistance “Rmetal_(sense)”of the bottom interconnect metal 405 _(B) may affect the voltage atlocal Kelvin sense nodes 306 _(ss). In each circuit unit 300, becausethe top of Rref 302 _(ss) and the top Rsense 301 _(ss), 303 _(ss), sharethe same Kelvin voltage, the ratio of currents in Rref and Rsense remainunchanged for each unit and, therefore, unchanged for an aggregate ofmultiple circuit units 300 _(ss). Therefore, for an implementation canbe achieved combining in parallel a plurality of circuit units 300 _(ss)as shown in FIG. 4, in order to reduce the net Rsense resistance value.

FIG. 5 is a second exemplary embodiment of the present invention for thecircuit as shown in FIG. 4. In this circuit 500, in order to negate theeffect of voltage drops through the resistance “Rmetal_(sense)” andRmetal_(top), the bottom interconnect 405 _(B) may be provided with aKelvin sense connection node 506 _(ss) to each circuit unit 300 _(ss).Thus, although Rmetal_(sense) and Rmetal_(top) affect the top voltage atthe first Kelvin connection node 306 _(ss), both R_(sense) andRmetal_(re) share the same Kelvin voltage at that node and so the neteffect is negated. The bottom Kelvin sense connection 506 _(ss) willhold a voltage in a negative feedback gain loop condition. An averagingnetwork—here shown as a set of resistors, Raverage 501 ₁, 501 ₂ . . .501 _(n), to which sense resistors 505 ₁, 505 ₂ . . . 505 _(N) areconnected via Kelvin sense connection nodes 506 ₁, 506 ₂ . . . 506 _(N),respectively, at the R_(average) resistors bottom interconnect 503—avoltage is provided that is the average at Kelvin sense point 501 _(ss)such that,V 503=(V 501-1+V 501-2+. . . V 501-n)/n.Again, the bottom Kelvin sense connection provides a connection point503 for the negative feedback gain loop as described hereinbefore.

One example of an implementation of the components of FIG. 5 may be tohave ten Rsense resistors of approximately one ohm each, Rref resistorsapproximately 1000 ohms each, and Raverage resistors about 200 ohmseach.

FIG. 6 is a simplified representation for FIG. 5. Referring now to bothFIGS. 5 and FIG. 6, a representative sum of all parallel sets ofreference resistors 302 _(ss), 304 _(ss) connected to each node 306_(ss) in FIG. 5 is illustrated as a single equivalent referenceresistor, Rref 602. Similarly, a single node 606 is representative ofall the nodes 306 _(ss) in FIG. 5, again where the current throughR_(ref) is small enough that the Rmetal_(ref) can be ignored. Thevoltage at node 603 in FIG. 6 is the same as the voltage at bottominterconnect 404 b in FIG. 5. The voltage drop across Rref 602 is theproduct of: the sum of the currents in each Rref in FIG. 5, and, theohmic value of Rref 602 in FIG. 6. The voltage at node 606 is the sum ofthe voltage drop across Rref 602 and the voltage at node 603. Thevoltage at node 501 in FIG. 6 is the average of the voltages at nodes506ss in FIG. 5. The OP AMP 101 in FIG. 6 is again illustrative of anygain component which forces the voltage at node 603 to be equal to thevoltage at node 506. Therefore, given only first order variation ofvoltages at nodes 506 _(ss) in FIG. 5, the current through Rref 602 andRsense 601 in FIG. 6 hold the same ratio as: Rref/Rsense.

FIG. 7 is an illustration of a first fundamental aspect in accordancewith the exemplary embodiment as shown in FIGS. 5 and 6. This embodimentshows an input node receiving current “Iset.” Sense resistor subunits701 _(ss) having sense resistor input terminals 706 _(ss) are connectedto the input node and have an output terminal 501 _(ss). In parallelwith said sense resistor subunit 701 _(ss), a reference resistor subunit702 _(ss), having an input terminal, connects to input terminal 706_(ss) and an output terminal 703 _(ss). The reference resistor subunit702 _(ss) is connected to said sense resistor input terminal at node 706_(ss) by a Kelvin connection. Via another Kelvin connection to node 501_(ss) and “averaging circuit” 711 is provided such that the average ofthe voltage at each node 501 _(ss) is the voltage for node 503, FIG. 5.The voltage at node 503 and at terminal 703 _(ss) provide connectionpoints for the negative feedback loop as described hereinbefore.

FIG. 8 is an illustration of a second fundamental aspect in accordancewith the exemplary embodiment as shown in FIGS. 5, 6, and 7. TheAveraging circuit is shown to further comprise a plurality of averagingresistors 713 _(ss).

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimelement in the singular is not intended to mean “one and only one”unless explicitly so stated. Moreover, no element, component, nor methodor process step in this disclosure is intended to be dedicated to thepublic regardless of whether the element, component, or step isexplicitly recited in the Claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. Sec. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for . .. ” and no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “comprising the step(s) of . . . ”

1. A current sense circuit device comprising: an input node; a senseresistor subunit having a sense resistor input terminal connected tosaid input node and having a sense resistor output terminal; connectedto the input node and said sense resistor subunit, a reference resistorsubunit, wherein said reference resistor subunit has an input terminalconnected to said input node and sense resistor input terminal via afirst Kelvin connection, and said sense resistor output terminal has asecond Kelvin connection; and connected to said second Kelvinconnection, an averaging circuit, having a plurality of input terminalsand an output terminal such that voltage at said averaging circuitoutput terminal is a voltage which is an average of voltages at all saidaveraging circuit input terminals and such that said reference resistoroutput terminal and said averaging circuit output terminal providerespective connections for a negative feedback gain loop.
 2. The deviceas set forth in claim 1, the averaging circuit further comprising:connected to said each second Kelvin connection of each of said sets, aresistor-based voltage averaging network.
 3. The device as set forth inclaim 1, said sense resistor subunit comprising: a set ofparallel-connected resistors providing a first net resistance and a setof series-connected reference resistors providing a second netresistance.
 4. The device as set forth in claim 2 wherein said secondKelvin connection connects to the averaging circuit.
 5. The device asset forth in claim 3 where net resistance ratio of said first netresistance to said second net resistance is at least approximately 4:1.6. A current sense resistor circuit with a negative gain feedback loopamplifier, said amplifier having an amplifier first input terminal andan amplifier second input terminal, the circuit comprising: saidamplifier first input terminal is connected for receiving a givenelectrical current; electrically connected to said amplifier first inputterminal, a plurality of parallel sense resistors, each having an inputterminal Kelvin connection and an output terminal Kelvin connection;electrically connected to said input terminal Kelvin connection, areference resistor means for establishing a reference currenttherethrough and having a reference resistor means output terminalconnection; and an averaging circuit having an averaging circuit inputterminal connected to said output terminal Kelvin connection and anaveraging circuit output terminal connected to said amplifier firstinput terminal, wherein a first voltage at said output terminal Kelvinconnection is provided to said averaging circuit input terminal and asecond voltage at said reference resistor means output terminalconnection is provided to said amplifier second input terminal.
 7. Thecircuit as set forth in claim 6 further comprising: connected to eachoutput Kelvin connection, a resistor averaging means for taking anaverage of all the voltages at said output Kelvin connection.
 8. In acontrol apparatus using electrical current sensing, an electricalcurrent sense subunit comprising: an input node for receiving a givenelectrical current to be sensed for said control apparatus; connected tosaid input node, a plurality of sense resistors, each of said senseresistors having an input terminal and an output terminal, said senseresistors being connected in parallel and each of said sense resistorshaving a substantially equal size wherein connection metal therebetweendoes not substantially add to net resistance thereof; connected to saidsense resistors via a Kelvin connection to said input terminal side ofeach of said sense resistors, at least one reference resistor whereinnet resistance ratio of said reference resistors to said sense resistorsis at least approximately 4:1; and connected to said output terminal ofsaid sense resistors, an averaging circuit such that use of averagevoltage across sense resistors and said reference resistor provides areduced-error averaging Kelvin sense potential.
 9. A current sensecircuit with averaging Kelvin sense elements, the circuit comprising:sensing resistor means for receiving an input current; connected via afirst Kelvin input connection to said sensing resistor means,referencing resistor means for receiving said input current; connectedvia a first Kelvin output connection to said sensing resistor means,means for providing an average voltage signal; and an operationalamplifier means for providing a negative gain feedback loop for saidcurrent sense circuit, having a first operational amplifier inputconnected to said referencing resistor means and a second operationalamplifier input connected to said means for providing an average voltagesignal.